This invention relates to the detection of an open circuit condition in input circuits, particularly in thermocouple inputs to temperature measuring circuits. One useful example of such a circuit is that disclosed in U.S. Pat. No. 4,115,785, issued Sept. 19, 1978, to the present inventor and several of his coworkers. In the use of the circuit of the above mentioned patent, it was found that the presence of noise signals in the thermocouple input circuits occasionally caused false indications of an open circuit condition. These false indications apparently were caused by noise signals overcoming the slight negative bias on amplifier 88, supplied by the d.c. supply -E at terminal 84 to make diode 92 conductive and thus provide the open circuit indication.
It is the object of this invention to provide an open circuit detector which will not be subject to false indications of an open circuit condition as a result of noise in the thermocouple input circuit.